Such a heat transfer tube in the heat-exchanger is generally contaminated on its inner surface with scale, (such as fur and rust) during operation, which must be cleanend off periodically to prevent decrease of efficiency.
Japanese Patent Publication No. 29929/69 discloses an automatic cleaning apparatus, in which a cleaning brush is placed in a receiving frame arranged at an end of the tube and is moved in the tube by switching the flowing direction of water. The apparatus of such a type has disadvantages, however, in that the flowing direction of water must be forcibly changed and the apertures provided on the receiving frame for the brush may deteriorate smoothness of the water flow.
Japanese Patent Publication No. 11662/74 discloses a method of removing scale, in which a linear and flexible cleaning body of streamer type is flowed in a jacket of spiral construction. In this case, however, complete removal of deposited scale is difficult because of flexibility of the cleaning body itself.
Japanese Opend Patent Application No. 158995/84 teaches a cleaning apparatus similar to the former one, in which a string is attached to the brush for preventing escape of the latter and for improving stability and reliability of the receiving frame. This measure can not eliminate the necessity of forcible switching of the water flow and the deterioration of smoothness of the water flow. Further, smoothness movement of the brush is inconsistent with higher resistance for facilitating removal of deposited scale, and the removed scale is located in front of the brush to inhibit its smooth movement.
Another method is known, in which spongy balls are floated in a heat-exchanging water and passed through the heat transfer tube to be cleaned. In this case, however, special equipment is necessary for placing and removing the spongy balls, resulting in a larger apparatus. Further, damage and wear of the balls must be determined for replacement with new ones, resulting in troublesome management. In order to solve these problems Japanese U. M. Publication No. 35584/77 has proposed such an apparatus of ball-recycling type, in which air in the balls is removed and replaced with sufficient water to provide an ideal cleaning condition, but which cannot solve the problems basically.
Accordingly, an object of the invention is to eliminate the above disadvantages and to ensure effective and automatic cleaning of each one of the heat transfer tubes arranged in parallel.